1. Field of the Invention
The present invention relates to a sensor apparatus, a control system having the sensor apparatus and an offset correction method of the sensor apparatus.
2. Description of Related Art
A method for diagnosing an acceleration sensor used for deploying an air bag installed in a vehicle is recited in, for example, Japanese Unexamined Patent Publication No. 2005-257304. Specifically, Japanese Unexamined Patent Publication No. 2005-257304 recites an air bag system, which includes acceleration sensing units and a control unit. Each acceleration sensing unit includes an acceleration sensing arrangement for sensing a collision of the vehicle. The control unit performs failure diagnosis of the acceleration sensing arrangement and a deployment operation of an air bag.
In the acceleration sensing unit, the acceleration sensing arrangement includes an acceleration sensor, a diagnosing device and a communication device. The acceleration sensor measures the acceleration. The diagnosing device outputs collision sensing information and diagnosis result information when the acceleration, which is greater than a preset collision determination threshold value, is sensed. The communication device outputs the diagnosis result information of the acceleration sensor.
The control unit includes a communication device and a failure diagnosing arrangement. The communication device receives the collision sensing information and the diagnosis result information, which are outputted from the acceleration sensing unit. The failure diagnosing arrangement includes an abnormality counter and a failure determining device. The abnormality counter is incremented based on the information received through the communication device. The failure determining device determines occurrence of a failure based on the counted number, which is counted by the abnormality counter.
In the above air bag system, the diagnosing device executes a test operation of the acceleration sensor. When an abnormality is found in the operation of the acceleration sensor, the diagnosing device outputs an abnormality code through the communication device. Then, when the failure diagnosing arrangement of the control unit receives the abnormality code through the communication device, the failure diagnosing arrangement increments the abnormality counter. Thereafter, when the number of the abnormality code counted with the abnormality counter reaches three, the failure diagnosing arrangement determines that the acceleration sensing unit has the failure. Therefore, the failure diagnosing arrangement flashes a warning light and stops the deployment control operation of the air bag on the acceleration sensing unit, which is determined to have the failure.
In the above system recited in Japanese Unexamined Patent Publication No. 2005-257304, the acceleration sensing unit outputs only the collision sensing information and the diagnosis result information besides the measured acceleration value. Thus, the control unit needs to determine the failure based on these outputs from acceleration sensing unit. In such a case, when the number of the acceleration sensing units installed in the vehicle 1 is increased, the load on the control unit is disadvantageously increased. Thus, the deployment operation of the control unit for deploying the air bag may possibly be disadvantageously affected.
Furthermore, in the acceleration sensing unit, the output value of the acceleration sensor may change with the temperature and with time. Thus, the offset correction for correcting the output value of the acceleration sensor by the amount of this change needs to be performed. However, in the above system, the acceleration sensing unit has no function for performing the offset correction on the acceleration sensing unit. Therefore, the offset correction of the output value of the acceleration sensing unit needs to be performed at the control unit. Therefore, the load on the control unit is also disadvantageously increased.